Iontophoresis is a drug delivery system used to facilitate the introduction of charged ions across the skin barrier by use of current. It provides the advantages of intravenous infusion withoout the problems associated with the use of needles. But there is poor understanding of the iontophoretic process itself. Intradermal microdialysis is a new technique which permits in vivo sampling of dermal extracellular fluid. The present study proposes to combine iontophoresis with microdialysis to measure the kinetics of transdermal delivery in health volunteers.